1. Field of the Invention
The present invention relates to all-optical 3R (retiming, reshaping and regenerating) regeneration of optical data pulses.
2. Description of the Related Art
In a current optical repeater, as described in a paper xe2x80x9cAll-optical 3R regeneration of 10 Gbps RZ optical data pulse with mode-locked laser diode devicesxe2x80x9d, H. Kurita et al, CPT Technical Digest, pages 139-140, a optical data pulse stream is split into a first input that is incident on a mode-locked laser diode and a second input that enters a light amplifier. The mode-locked laser diode has a gain region with a basic mode-locked oscillation frequency and a saturable absorptive region. By the lasing oscillation of the diode, the diode""s oscillation frequency is pulled into the clock frequency of the data pulse stream so that the clock timing of the data pulse stream is extracted and a stream of constant-amplitude output pulses precisely aligned in phase with the input pulses can be obtained as a low-noise optical clock pulse stream. This low-noise clock pulse stream is delayed so that its pulse timing is aligned with the output pulse of the light amplifier. On the other hand, the data stream that is incident on the light amplifier is sufficiently amplified by the light amplifier in relation to the intensity of the delayed clock pulse stream so that the data pulse dominates the on-off control of an optical gate, or a semiconductor waveguide saturable absorber.
If the output of a mode-locked laser diode has the same wavelength as that of the input, undesired disturbance would result. More specifically,.since the diode has a small internal loss, energy leaving the absorptive region of the device would be subject to induced amplification in the gain region and circulate through the resonator. As a result, the gain and absorption characteristics of the device are randomly modulated. In order to avoid such a disturbance, the current mode-locked laser diode is designed to produce an output that is different in wavelength from the input.
Thus, the delayed clock pulse has a different wavelength from the output of the light amplifier. Because of their different wavelengths, they are coupled together by an optical coupler. The combined optical signal is entered into the optical gate through one of its ports, where the clock pulse stream is patterned after the amplified data pulse and delivered from the other port of the optical gate. An optical filter is coupled to the output of the optical gate to eliminate components other than the modulated clock pulse stream.
Optical repeaters are used extensively in wavelength division multiplex optical communication systems. However, wavelength division multiplexing requires in most cases that the output of an optical repeater be the same in wavelength as that of the incident optical signal. Therefore, a need does exist to develop optical repeaters capable of producing the same wavelength as that of the incident signal.
It is therefore an object of the present invention to provide an optical repeater for regenerating data pulses of wavelength equal to the wavelength of input data pulses.
The stated object is obtained by the use of cascaded first and second mode-locked laser diodes operating in a passive synchronization mode at different wavelengths for clock extraction from an optical data pulse stream. The first mode-locked laser diode produces coherent clock pulses of wavelength different from the input data pulse stream and the second mode-locked laser diode produces coherent clock pulses of wavelength equal to the input data stream.
According to the present invention, there is provided an optical repeater comprising an optical splitter for splitting an optical data pulse stream of first wavelength into first and second components, and a first mode-locked laser diode connected to receive the first component of the optical data pulse stream from the optical splitter for producing a first clock pulse stream of second wavelength different from the first wavelength, the first clock pulse stream having a frequency synchronized to the optical data pulse stream. A second mode-locked laser diode is connected to receive the first clock pulse from the first mode-lock laser diode for producing a second clock pulse stream of wavelength equal to the first wavelength, the second clock pulse stream having a frequency synchronized to the first clock pulse stream. A light amplifier amplifies the second component of the optical data pulse stream from the optical splitter. An optical modulating device modulates the second clock pulse stream with the amplified data pulse stream to produce an output data pulse stream of retimed and reshaped pulse waveform.